There are many well-known continuous separation processes in industry using selective adsorption of at least one component among several in a mixture of fluids, notably chromatography processes referred to as simulated countercurrent processes using the property of porous solids, in the presence of liquid, gaseous or supercritical mixtures, to retain more or less significantly the various constituents of the mixture.
Separation or fractionation processes based on chromatography are most often implemented in a system comprising a series of columns or fractions of columns interconnected in series and forming a loop. A porous solid of determined grain size constitutes the stationary phase. The mixture F to be separated is fed into the loop, then displaced by means of a carrier fluid, eluent E, and the various constituents flow out successively according to whether they are retained more or less significantly by the stationary phase. Injection points for the mixture F to be separated and the solvent or eluent E, and fluid extraction points, for extract EX and raffinate RA, delimiting different zones, are distributed along this loop. The number of zones can vary but the separation systems most often comprise four main zones.
In simulated moving bed separation processes, the solid phase is placed in a certain number n of fixed beds (generally 4.ltoreq.n.ltoreq.24) arranged in series and it is the concentration profile that is displaced at a substantially uniform speed around a closed loop by successive shift of the injection and extraction points by means of a rotary valve or more simply of a series of suitably controlled on-off valves. This circular shifting, at each period T, of the various liquid inflows-outflows in a given direction allows to simulate displacement of the solid adsorbent in the other direction. The main liquid inflows are the flow of feed F and the flow of eluent S. According to the configuration selected, according to whether a recycling pump is used or not in the cycle, the outflows of extract EX or of raffinate RA can be controlled by the pressure at the inlet of the recycling pump or by means of flowmeters.
Simulated moving bed chromatography processes are for example described in patents EP-0,415,822; 0,568,407; FR-2,699,917; 2,704,158, and in patent applications FR-97/05,485 or 97/07,756.
In industrial chromatography processes, one of the main parameters allowing selective adsorption of the species to be separated and/or their solubility in the eluent is the polarity of the eluent. Most often, the eluent allowing optimum productivity of the process is a binary mixture of two solvents of different polarities.
For good stability of the working point of the loop, it is important that the fluid mixture (eluent) used as a carrier fluid for the constituents of the feed in the separation loop has a well-defined composition, where the proportion of each solvent remains really constant. Now, the eluent is generally recycled by evaporation and/or distillation at the outlet of the separation system. The solvents forming the eluent often have different volatilities. In the case of non-ideal mixtures in the presence of an azeotrope, the composition of the eluent may also be different from that of the azeotrope. The composition of the mixture recycled after use is consequently modified. It is therefore necessary to restore the set composition and to be able to perform continuous measurement of its variations.